This study is testing a new drug called ALN-SNCA for people with early Parkinson's disease (PD). PD is a problem with the nervous system that makes it hard to move because the brain's nerve cells are damaged by a protein called α-synuclein. This study wants to see if the new drug is safe, what side effects it might have, and if it can lower the bad protein levels in the fluid around the brain and spine (CSF). The study checks how much of the drug is in the blood, urine, and CSF at different times. The aim is also to see if the drug can slow down PD symptoms.

• The study involves visits to the clinic for tests and samples of blood, urine, and CSF.
• Participants must have had PD for less than 4 years and either not be on standard PD treatment or be stable on their current treatment.
• Participants should not have other medical conditions that could confuse the study results or increase risks.

Note: Other rules for joining or not joining the study may apply.